Technique for underfilling stacked chips on a cavity MLC module

ABSTRACT

An electronic device is provided comprising a stacked integrated circuit chip assembly wherein the top chip of the assembly is solder connected to the surface of an interconnection substrate with the other chips of the assembly being enclosed in a cavity in the interconnection substrate wherein the cavity and electrical connections of the assembly and between the substrate and top chip are sealed by supplying an encapsulant to the cavity through a through opening in the substrate which communicates with the cavity.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to chip containing electronic devices and,in particular, to electronic devices containing an assembly of aplurality of electrically connected stacked chips wherein typically alarger top chip of the assembly is electrically connected to amulti-layer ceramic (MLC) cavity substrate and a smaller lower chip orchips of the assembly are enclosed in the cavity of the substrate and toa method of their manufacture.

2. Description of Related Art

Electronic components utilizing integrated circuit chips are used in anumber of applications. Controlled Collapse Chip Connection is aninterconnect technology developed by IBM as alternative to wire bonding.This technology is generally known as C4 technology or flip chippackaging. Broadly stated, one or more integrated circuit chips aremounted above a single or multi-layer substrate and pads on each chipare electrically connected to corresponding pads on the substrate by aplurality of electrical connections such as solder bumps. The integratedcircuit chips may be assembled on the substrate in a solder bump arraysuch as a 10×10 array. The chip containing substrate is then typicallyelectrically connected to another electronic device such as a circuitboard by pin connectors with the total package being used in anelectronic device such as a computer.

Flip chip packaging is described in U.S. Pat. No. 5,191,404 which patentis hereby incorporated by reference. In general, flip chip joining isdesirable for many applications because the footprint or area requiredto bond the chip to the substrate is equal to the area of the chipitself. Flip chip joining also exploits the use of a relatively smallsolder bump which typically measures a height of approximately 1 mil to1.5 mils and a width of approximately 2 to 4 mils to join the pads onthe chip to corresponding pads on the substrate. Electrical andmechanical interconnects are formed simultaneously by reflowing thebumps at an elevated temperature. The C4 joining process isself-aligning in that the wetting action of the solder will align thechip's bump pattern to the corresponding substrate pads. This actioncompensates for chip to substrate misalignment up to several mils whichmay be incurred during chip placement.

In the joined flip chip package there is necessarily an opening or spacebetween the pad containing surface of the integrated circuit chip andthe pad containing surface of the joined substrate resulting from thethickness of the pads on each surface and the solder bump connectionbetween the pads. This open space can not be tolerated because anyinterference with the solder connections will adversely affect theperformance of the package. For example, moisture, the infusion ofthermal paste used to increase heat transfer from the chip and themechanical integrity of the chip due to the possible breaking of thesolder bump electrical connections are all serious problems. To solvethese problems, the solder bumps of the joined integrated circuit chipsand substrate are typically encapsulated totally or a sealant is usedaround the chip edges to seal the joined opening.

The encapsulation of integrated circuit chips bonded to substrates toimprove their reliability is well known. For non C-4 joining, chips wirebonded or tap bonded are typically completely encapsulated in a transfermolded thermoset or thermoplastic polymer. Basically, this processinvolves melting the polymer in a cavity within the mold. A plunger thenrams the molten polymer through an orifice into the mold ventricle. Theintegrated circuit chip and substrate are bonded to each other using apolymeric adhesive and the package is placed in the mold and the moltenpolymer forces in and around the package to totally encapsulate thedevice.

Flip chip bonding offers many advantages in electronics manufacturecompared to the complete encapsulation techniques above and one of themost important is the ability to remove and replace the chip withoutscrapping the substrate. This removal of the chip by heating and liftingof the chip from the substrate and replacement with typically a new chipis termed rework and can be performed numerous times without degradingthe quality or reliability of the reworked electronic component.

Encapsulation of the flip chip packages however presents rework andother problems. The flip chip package must also be reliable and thermalmismatches between the encapsulant, chip, substrate and/or solder bumpsmust be minimized to avoid stressing and damaging of the package. Theencapsulant must also be able to be heated and softened for the lift-offprocedure.

Recent developments in electronic component fabrication now providecomponents utilizing an assembly of stacked chips, instead of a singlechip, mounted to a substrate. In general, a plurality of chips are C-4bonded in a stack assembly resulting in corresponding spaces betweeneach of the bonded chips. Typically, the chips are of about the samesize (width and length and surface area) and are mounted to a top chiphaving a larger width and length and surface area which larger chip hasperipheral non-bonded pads and forms the top of the stacked assembly.Once the chips are joined in the assembly, the peripheral non-bondedpads of the top chip of the stacked assembly are then C4 joined to anMLC substrate. This substrate has a cavity to accommodate the smallerconnected stacked chips and the uppermost top stacked chip overlies theperiphery of the cavity. The peripheral non-bonded C4 bumps on the topchip are then C4 bonded to the surface of the substrate with the smallerstacked chips being positioned and enclosed in the cavity.

The conventional chip underfill process to encapsulate the space betweena single chip bonded to a non-cavity substrate surface typicallypositions the bonded chip above the top of the substrate and thenapplies the underfill material to the substrate adjacent to theperiphery of the chip to be underfilled. Capillary action draws theunderfill encapsulated material into the space between the chip and thesubstrate to form a void free filled space between the chip and thesubstrate. This technique works very well for a single chip attached tothe surface of a substrate but is not reliable for stacked chipassemblies wherein the stacked chips are enclosed in a cavity of asubstrate.

Bearing in mind the problems and deficiencies of the prior art, it istherefore an object of the present invention to provide a stackedintegrated circuit chip assembly comprising a plurality of electricallyconnected chips with the chip assembly being electrically connected toan interconnection substrate forming an electronic package whereinperipheral non-bonded pads on the uppermost top chip of the assembly areelectrically connected to pads on the interconnection substrate with thestacked lower chip or chips of the assembly being enclosed in a cavityin the interconnection substrate with the solder connections between thestacked chips and cavity area being effectively sealed (encapsulated) toprovide mechanical, electrical and chemical reliability for theelectronic package.

It is another object of the present invention to provide a method formaking an electronic component comprising a stacked integrated circuitchip assembly comprising a plurality of electrically connected chipselectrically connected to a substrate, the component package havingenhanced electrical, mechanical and chemical reliability propertieswherein non-electrically connected peripheral pads on the top uppermoststacked chip of the assembly are electrically connected to pads on thesurface of the interconnection substrate by solder connections with thelower chip or chips of the assembly being enclosed in a cavity in thesubstrate and the solder connections between the stacked chips and thecavity encapsulated with an encapsulant.

Still other objects and advantages of the invention will in part beobvious and will in part be apparent from the specification.

SUMMARY OF THE INVENTION

In one aspect of the invention an electronic device is provided havingenhanced mechanical, electrical and chemical reliability comprising anassembly of a plurality of stacked electrically connected integratedcircuit chips having a top chip and a bottom chip, the top chip having alarger width and length and a larger surface area than the other chipsin the assembly and having peripheral non-bonded conductive pads thereonwhich peripheral pads are electrically connected to correspondingconductive pads on the surface of an interconnection substrate by solderconnections between the corresponding sets of pads, the interconnectionsubstrate having a cavity having an open area smaller than the surfacearea of the top chip and an open area larger than the other chips whichcavity accommodates and encloses the bottom chip and other chips of theassembly other than the top chip and wherein the cavity and solderconnections between the chips are filled with an encapsulant bysupplying an encapsulant material to the cavity through a throughopening in the substrate which communicates with the cavity, the openingextending from a surface of the substrate to the cavity.

In the method of the invention to make the electronic device, theencapsulant is typically heated and liquified and caused to flow throughthe opening into the cavity and the encapsulant flows into the cavityand spaces between the stacked chips and between a space between theperiphery of the top chip and the substrate surface preferablysubstantially filling the cavity and encapsulating all of the solderbump connections including the peripheral solder connections of the topchip providing a mechanically, electrically and chemically stabilizedand sealed stacked chip assembly containing electronic device.

In yet another aspect of the present invention, a method is provided formaking an electronic component comprising an assembly of a plurality ofelectrically connected stacked integrated circuit chips and aninterconnection substrate, wherein peripheral conductive pads on a topchip of the stacked assembly are electrically connected to correspondingpads on the surface of the interconnection substrate by solderconnections between their corresponding pairs of pads the methodcomprising the steps of:

providing an integrated circuit chip electrically connected stackedassembly containing a plurality of chips wherein the top chip of theassembly contains peripheral pads which are electrically connected tocorresponding pads on the surface of an interconnection substrate by aplurality of solder connections forming a space between the padcontaining surface of the top chip and the pad containing surface of thesubstrate with the other chips of the stacked assembly being enclosed ina cavity in the substrate;

providing a fluid encapsulant; and

supplying the encapsulant to the cavity through a through opening in thesubstrate extending from a surface of the substrate and communicatingwith the cavity and sealing the cavity and encapsulating the solderconnections between the stacked chips and the space between the top chipand the interconnection substrate with the encapsulant.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention believed to be novel and the elementscharacteristic of the invention are set forth with particularity in theappended claims. The figures are for illustration purposes only and arenot drawn to scale. The invention itself, however, both as toorganization and method of operation, may best be understood byreference to the detailed description which follows taken in conjunctionwith the accompanying drawings in which:

FIG. 1 is a cross-sectional view of an electronic component comprising acavity containing substrate to which substrate a stacked chip assemblycomprising a bottom chip and an electrically connected larger top chipis electrically attached, with the bottom chip enclosed within thecavity and peripheral pads on the top chip being electrically connectedto the surface of the substrate and the cavity and electricalconnections sealed.

FIG. 2A is a side view of a stacked chip assembly as in FIG. 1 ready forelectrical attachment to a cavity containing substrate.

FIG. 2B is a side view of a stacked chip assembly comprising a largerchip electrically connected to an intermediate chip, which intermediatechip is electrically connected to another chip.

FIG. 3 is a perspective view of an electronic component of the prior artcomprising an integrated circuit chip containing pads and solder bumpswhich chip is to be electrically joined to corresponding pads on anon-cavity interconnection substrate.

FIG. 4 is a cross-sectional view of FIG. 3 along lines 4—4 after thechip is joined to the substrate showing the chip and substrateelectrical interconnections being totally encapsulated.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

In describing the preferred embodiments of the present invention,reference will be made herein to FIGS. 1-4 of the drawings in which likenumerals refer to like features of the invention. Features of theinvention are not necessarily shown to scale in the drawings.

Referring first to FIGS. 3 and 4, a typical prior art non-cavityelectronic component 10 is shown comprising integrated circuit chip 11and interconnection substrate 15. Integrated circuit chip 11 is shownhaving conductive pads 12 overlaid with solder bumps 13. Correspondingconductive pads 14 are shown on substrate 15. Referring to FIG. 4,electronic component 10 is depicted in cross-section wherein integratedcircuit chip 11 is solder connected to interconnection substrate 15. Thechip 11 is electrically connected to the substrate 15 by a plurality ofsolder connections 13 such as solder bumps, in a method known as C4 orflip chip packaging. The lower surface of substrate 15 may containconnectors such as pin connectors 23 for connection of the substrate 15to another electronic device such as a circuit board.

The solder interconnections 13 of electronic component 10 as shown inFIG. 4 are encapsulated by a thermoplastic polymer or other encapsulant16 shown filling the space 19 between pad containing surface 17 of chip11 and pad containing surface 18 of substrate 15. The encapsulant 16 isshown both around the periphery of chip 11 and under the chip totallyencapsulating all the solder bump 13 connections.

FIG. 1 shows a stacked chip assembly 25, as shown in FIG. 2A,electrically connected to a cavity containing substrate 20.

Referring to FIGS. 1 and 2A, the integrated circuit chips 26 and 30 usedto form the stacked assembly 25 may be any of a number of integratedcircuit devices such as passive devices or a very large scaleintegration (VLSI) or ultra large scale integration (ULSI) activedevices. Exemplary devices are static random access memory (SRAM),dynamic (DRAM), microprocessor and ASIC chips and combinations thereof.

Interconnection substrate 20 is shown in FIG. 1 having a cavity 21 andmay be constructed of any number of suitable substrate materials such asceramic or metal. Alumina, glass ceramic, and the like are exemplary.Interconnection substrate 20 is typically a multi-layer substrate. It ispreferable to use similar materials for both the chips 30 and 26 ofassembly 25 and substrate 20 to minimize thermal mismatch for the formedelectronic component and to enhance the integrity of the solder bumps 32bonding the chip assembly 25 to the substrate 20, which bumps mightotherwise be damaged due to thermal expansion when the electroniccomponent is on.

Chip assembly 25 is electrically connected to substrate 20 by flip chipC4 joining which involves aligning the pads 31 and solder bumps 32 ofchip 26 of stacked assembly 25 with the pads 33 of substrate 20 and thenjoining the chip 26 of the stacked assembly 25 and the substrate 20 byheating to melt the solder bumps 32 and forming an electrical andmechanical connection between the chip 26 (and assembly 25) andsubstrate 20. This process is usually called solder reflow.

Stacked chip assembly 25 as shown in FIG. 2A is first fabricated beforeattachment to substrate 20 by solder bonding bottom chip 30 to top chip26 by aligning pads 27 and 29 and solder bumps 28 and reflowing thesolder. This procedure is shown schematically in FIG. 3 where bumpcontaining pads on chip 11 are bonded to pads on substrate 15. In thiscase, the substrate 15 is chip 26. The stacked assembly 25 is thensolder joined to substrate 20 as described hereinabove.

As shown in FIG. 1, top chip 26 is larger (in width and length andsurface area) than bottom chip 30 and overlies the periphery or sidewalls 21 b and 21 c of cavity 21. Chip 30 is shown surrounded andenclosed in cavity 21. Encapsulant 35 is shown being forced into cavity21 from dispenser 24 through inlet tube 24 a which extends into throughopening 22. Through opening 22 extends from surface 40 of substrate 20to wall 21 a into cavity 21. The encapsulant 35 fills the cavity 21 andthe space 41 between chip 30 and chip 26 and between the space 42between chip 26 and substrate 20. The encapsulant 35 is shown extendingto the periphery of chip 26.

Once the stacked chip assembly 25 has been electrically connected tosubstrate 20, the substrate 20 can be electrically connected to anotherelectronic device by processes well known in the art such as card join.This is accomplished by a variety of input/output methods. Pin gridarrays, such as 23 shown on FIG. 1, Ball grid arrays, wire or castcolumn arrays are all connection strategies to second level packagingwhich may be used.

The materials used for encapsulation may be high temperaturethermoplastic resins and may be selected from a number of resins havingthe properties necessary to provide the desired C4 encapsulation andmechanical, electrical and chemical reliability characteristics of theelectronic components of the invention. In general, the encapsulant mustnot degrade, have a suitable glass transition temperature and viscositycharacteristics for filling the cavity and be capable of reworking. Itis also important that the resins be soluble in solvents such asN-methylpyrolidone (NMP) for applying the resin to the C4 assembly andfor reworkability of the sealed C4 assembly. Other solvents includecommon aldehydes, ketones, tetrahydrofuran, HFIP andgamma-butyrolactone. An exemplary encapsulant is a modified epoxy.

Fillers have been used in encapsulants of the prior art to reduce and/orcontrol the coefficient of thermal expansion or control the flow of theencapsulant during application and drying. This is important to minimizecracking or other problems caused by uneven thermal expansion of thechip, substrate, solder bump and encapsulant during use of theelectronic component and such fillers may be used in the presentinvention.

Filled resins may also be employed for special situations where it isdesired to, for example, control the viscosity of the resin during theencapsulation process. Any of the usual fillers may be used such assilica, ceramic, glass/ceramic, barium titanate, alumina, Kevlar, boron,carbon and PBI fibers in an amount typically of, by weight, about 0.1 to0.5% or more.

The encapsulation process is performed by melting or liquifying theencapsulant 35 and forcing the encapsulant into cavity 21 throughthrough opening 22 in substrate 20. A dispensing tool 24 having aninjection tube 24 a may be used to force the encapsulant into thecavity.

Referring again to FIG. 1, a through opening 22 is provided in substrate20 from the lower wall 21 a of cavity 21 to the surface 40 of thesubstrate. The through opening 22 preferably extends from the lower wall21 a of the cavity to the surface 40 of the substrate which surface isnot being joined to the stacked chip assembly. A dispenser 24,preferably having a elongated tube 24 a, is inserted into opening 22 andis used to dispense a paste or encapsulant 35 into cavity 21. Thisprocess is performed after stack assembly chip 25 has been solderattached to substrate 20.

The through opening 22 is preferably positioned near (adjacent) theperiphery or side walls 21 b or 21 c of cavity 21 and preferably extendsin a straight line from the surface 40 of the substrate to the cavity21. The through opening 22 at this position has been found to allowcapillary action to draw the encapsulant or underfill material into thespaces between the chips before the cavity is filled. The throughopening 22 may also be provided in cavity side walls 21 b or 21 c orcentrally in wall 21 a, however this is not as preferred as the throughopening 22 at or near the periphery of cavity 21.

The use of a through opening 22 in substrate 20 has also been found toprovide a number of other significant advantages for underfilling astacked chip assembly in a substrate having a cavity. For one, thethrough opening 22 allows for the expansion of gases in the cavityduring reflow without disturbing connections of the larger chip 26 tothe substrate 20. Typically, the space 42 between the chip and thesubstrate may be filled with flux and prevent gases from escaping andwithout the through hole, the expanded gases can force the chip off thesurface of the substrate during reflow and result in a poor ornon-existing connection.

For dispensing the underfill material 35, it is preferred that thesubstrate 22 be positioned with the cavity and connected chip assemblydownward. Accordingly, as shown in FIG. 1, the chip assembly 25 isplaced downward with the substrate surface 40 being on top. This ispreferred so that the space between the chips and the C4 areas arefilled void free before the remainder of the larger volume cavity isfilled. As noted hereinabove, in a conventional process forencapsulating a single chip bonded to a substrate, the encapsulant wouldbe dispensed with the chip in an upward position. Using this typeprocess the encapsulant may wick between the large chip and thesubstrate but may also wick between the substrate and the small chipbefore the space between the chips is filled. This creates an air pocketwhich adversely affects the reliability of the connection.

Through opening 22 in substrate 20 may be formed in any convenientmanner. It can be formed into the substrate prior to firing, but ispreferred to be machined into the ceramic after firing. It can also beultrasonically machined in the ceramic after firing. The diameter of thethrough opening 22 may vary widely and is typically about 0.5 to 3 mm indiameter. A typical substrate 20 may range from about 21 mm wide to 64mm on each side and 2 mm to 10 mm thick with the cavity opening about 5mm to 22 mm on each side and 1 mm to 3 mm deep. The depth of cavity 21(height of sidewalls 21 b and 21 c) and the length of lower wall 21 aare sufficient to accommodate the stacked chips without contact withsidewalls 21 b or 21 c of lower wall 21 a and will vary depending on thenumber of stacked chips in the stack chip assembly 25. Typically thecavity 21 will allow for about 0.25 mm to 1 mm clearance between thesidewalls 21 b and 21 c of the cavity and the stacked chips and about0.2 mm to 0.5 mm clearance between the lower wall 21 a and the top ofthe stacked chip. The larger chip 26 which is electronically attached tosubstrate 20 will overly the periphery of the cavity 21 and typically beabout 2 mm to 10 mm larger than the opening of the cavity 21.

In FIGS. 1 and 2A only one chip 30 is shown stacked on larger chip 26and accordingly the cavity is sized to fit the one chip 30. A pluralityof chips can be stacked on larger chip 26 as is known in the art. Such astacked chip assembly is shown in FIG. 2B where another chip 43 iselectrically connected to chip 30 by pads 37 and 39 and solder bump 38.

Referring now to FIG. 2A, a stack chip assembly 25 is shown ready forelectrical connection to a substrate 20 as shown in FIG. 1. Thus, asmaller chip 30 is shown attached to a larger chip 26 by solderconnections 28 between bonding pads 27 of chip 26 and bonding pads 29 ofchip 30. Larger chip 26 has peripheral bonding pads 31 with solder bumps32 attached thereto. When chip assembly 25 is to be electricallyconnected to substrate 20, the solder bumps 32 of chip 26 are alignedwith pads 33 of substrate 20 and the solder reflowed joining chip 26 ofstack chip assembly 25 to substrate 20. The stacked chip-substrateassembly is then ready for underfill as shown in FIG. 1 using the methodof the invention as described hereinabove.

While the present invention has been particularly described, inconjunction with a specific preferred embodiment, it is evident thatmany alternatives, modifications and variations will be apparent tothose skilled in the art in light of the foregoing description. It istherefore contemplated that the appended claims will embrace any suchalternatives, modifications and variations as falling within the truescope and spirit of the present invention.

Thus, having described the invention, what is claimed is:
 1. A methodfor making an electronic component comprising an assembly of a pluralityof electrically connected stacked integrated circuit chips and aninterconnection substrate, wherein peripheral conductive pads on a topchip of the stacked assembly are electrically connected to correspondingpads on the surface of the interconnection substrate by solderconnections between their corresponding pairs of pads the methodcomprising the steps of: providing an integrated circuit chipelectrically connected stacked assembly containing a plurality of chipshaving a top chip and a bottom chip wherein the top chip of the assemblyhas a larger width and length and a larger surface area than other chipsin the assembly and contains peripheral pads which are electricallyconnected to corresponding pads on the surface of an interconnectionsubstrate by a plurality of solder connections forming a space betweenthe pad containing surface of the top chip and the pad containingsurface of the substrate with the other chips of the stacked assemblybeing enclosed in a cavity in the substrate, the cavity having an openarea smaller than the surface area of the top chip and an open arealarger than the other chips, which cavity accommodates and encloses thebottom chip and other chips of the assembly other than the top chip;providing a fluid encapsulant; and supplying the encapsulant to thecavity through a through opening in the substrate extending from asurface of the substrate and communicating with the cavity and sealingthe cavity and encapsulating the solder connections between the stackedchips and the space between the top chip and the interconnectionsubstrate with the encapsulant.
 2. The method of claim 1 wherein thethrough opening extends from the bottom wall of the cavity to theinterconnection substrate surface.
 3. The method of claim 2 wherein thethrough opening is straight.
 4. The method of claim 3 wherein thethrough opening is at or near the periphery of the cavity.
 5. The methodof claim 1 wherein two or more stacked chips are electrically connectedto the top chip.